In recent years, electron sources using cathodes with monocrystalline tungsten needle electrodes having coating layers of zirconium and oxygen (hereinafter referred to as ZrO/W electron sources) have been used to obtain electron beams that are brighter and have a longer operating life than hot cathodes (see Non-Patent Document 1).
ZrO/W electron sources are obtained by providing a reservoir consisting of zirconium and oxygen on a needle cathode composed of tungsten monocrystals having an axial orientation in the <100> orientation, so that the zirconium and oxygen are diffused to form a coating layer (hereinafter referred to as a ZrO coating layer), said ZrO coating layer reducing the work function of the (100) plane of the tungsten monocrystals from 4.5 eV to about 2.8 eV, so that only the tiny crystalline facet corresponding to the (100) plane formed at the tip of this cathode forms an electron emission region, as a result of which an electron beam that is brighter than that of conventional thermionic cathodes can be obtained, and the operating life is also prolonged.
As shown in FIG. 1, a ZrO/W electron source comprises a needle cathode 1 of tungsten in the <100> orientation for emitting an electron beam attached by welding or the like at a predetermined position on a tungsten filament 3 provided on a conductive terminal 4 anchored to insulator 5. A reservoir 2 of zirconium and oxygen is formed in a portion of the cathode 1.
The tip portion of the cathode 1 of the ZrO/W electron source is positioned between a suppressor electrode and an extractor electrode for use. The cathode 1 is electrically heated to about 1800 K by means of the filament 3, and generally, a high negative voltage is applied to the cathode 1 against the extractor electrode, while a negative voltage of a few hundred volts is applied to the suppressor electrode against the cathode 1, suppressing thermal electrons issuing from the filament 3.
While ZrO/W electron sources are widely used in critical dimension SEM and wafer inspection equipment at low accelerating voltages, when observing SEM images at relatively high optical system magnifications, vibrations from around the equipment can generate noise, thus reducing the resolution, sometimes to such a degree that measurements are not possible. This noise is known to be caused by vibration of the tungsten filament of the ZrO/W electron source at a resonance frequency.    Non-Patent Document 1: D. Tuggle, J. Vac. Sci. Technol., 16, p. 1699 (1979).